Eperimental studies on the structure of the lower mantle and themantle fractionation

测定了Mg_2SiO_4-Fe_2SiO_4-Fe_2SiO_4系橄榄石-尖晶石后相平衡。结果表明，镁铝尖晶石(Fe/Mg+Fe&lt；0.2)在很窄的压力范围内(1600℃时为-lt；0.15 Gpa)分解为钙钛矿和镁闪锌矿。离解压力几乎与铁含量无关，与650公里处的离解压力一致。这些结果表明，650公里不连续面的尖锐可能确实是由于橄榄岩地幔中的这种解离。将尖晶石分解的相界指定为反射波研究所隐含的尖锐不连续面，估算出过渡带底部(655公里深度)的温度约为1600C。考虑到CaSiO_3-MgSiO_3-Al_2O_3体系中已证实的镁橄榄岩和地幔橄榄岩体系中已证实的镁闪石分解，建立了下地幔以下的矿物学模型。熔融实验m…在2000-3000℃下，对可育橄榄岩进行了更多的ENT，最高可达2.5 Gpa。在下地幔条件下，富镁钙钛矿是液相线。以镁硅比、铝硅比、钙硅比和钙铝比为约束条件，通过少量的镁硅钙钛矿分馏(10-20wt%)，可以从球粒陨石来源中提取原始上地幔成分。其结构特征为八配位八面体中含有镁的SiO_6八面体的三维骨架，晶胞压缩主要受SiO_6八面体的倾斜控制。利用布里渊光谱首次测定了常压和常温下钙钛矿结构的单晶弹性模数。我们的弹性数据与适用于下地幔的地震模型的比较表明，只有当镁硅酸盐钙钛矿的剪切模数具有很强的负温度导数时，岩石学模型才能与实验数据相一致。较少

The olivine( )-modified spinel( )-spinel(y)-postspinel phase equilibria in the system Mg_2SiO_4-Fe_2SiO_4 were determined up to 26 GPA and 1600 C. It was demonstrated that the magenesian spinel (with Fe/Mg+Fe<0.2) dissociates into perovskite and magnesiowustite within an extremely narrow pressure interval (<0,15 GPa at 1600 C). The dissociation pressure is almost independent of iron content and coincide to that at 650km. These results indicate that the sharpness of the 650km discontinuity may indeed be due to this dissociation in peridotitic mantle. Assigning the phase boundaries of the spinel dissociation to the sharp discontinuity implied by study of reflected waves, the temperature at the base of the transition zone (655km depth) is estimated to be about 1600 C. Taking the dissociation of the majorite dissociation confirmed in the system CaSiO_3-MgSiO_3-Al_2O_3 and mantle peridotites into consideration, the mineralogical model was constructed down to the lower mantle.Melting experim … More ents on a fertile peridotite was performed up to 2.5 GPa and at temperatures 2000-3000 C. The MgSiO_3-rich perovskite was found to be the liquidus phase at the lower mantle conditions. Using the Mg/Si, Al/Si, Ca/Si, and Ca/Al ratio as constraints, it was shown that primitive upper mantle compositions can be derived from a chondritic source through a small degree of MgSiO_3-perovskite fractionation (10-20wt%).Crystal structure of MgSiO_3perovskite was determined at high pressures up to 9.6GPa as well as at atmospheric pressure, using single crystals. The structure was characterized by the three dimensional framework of SiO_6 octahedra with Mg in the eight-fold coordinated cuboctahedron and the unit cell compression is controlled dominantly by the tilting of SiO_6 octahedra. The singly-crystal elastic module of MgSiO_3 perovskite was determined for the first time under atmospheric pressure and room temperature using Brillouin spectroscopy. Comparison of our elasticity data with seismic models appropriste to the lower mantle reveals that petrological models having either pyrolite or pyroxene stoichiometries are compatible with the experimental data only if the shear modulus of MgSiO_3 perovskite has a very strong negative temperature derivative. Less